Surface Engineering of a 3D Topological Network for Ultrasensitive Piezoresistive Pressure Sensors

Flexible Piezoresistive Sensors Based on PPy Granule-Anchored Multilayer Fibrous Membranes with a Wide Operating Range and High Sensitivity

The employment of flexible piezoresistive sensors has sparked growing interest within the realm of wearable electronic devices, specifically in the fields of health detection and e-skin. Nevertheless, the advancement of piezoresistive sensors has been impeded by their limited sensitivity and restricted operating ranges. Consequently, it is imperative to fabricate sensors with heightened sensitivity and expanded operating ranges through the utilization of the appropriate methodologies. In this paper, piezoresistive sensors were fabricated utilizing electrospun polyvinylidene fluoride/polyacrylonitrile/polyethylene-polypropylene glycol multilayer fibrous membranes anchored with polypyrrole granules as the sensing layer, while electrospun thermoplastic polyurethane (TPU) fibers were employed as the flexible substrate. The sensitivity of the sensor is investigated by varying the fiber diameter of the sensing layer. The experimental findings reveal that a concentration of 14 wt % in the spinning solution exhibits high sensitivity (996.7 kPa-1) within a wide working range (0-10 kPa). This is attributed to the favorable diameter of the fibers prepared at this concentration, which facilitates the uniform in situ growth of pyrrole. The highly deformable TPU flexible fibers and multilayer sensing layer structure enable different linear responses across a broad pressure range (0-1 MPa). Furthermore, the sensor demonstrates good cyclic stability and can detect human movements under different pressures. These results suggest that the piezoresistive sensor with a wide operating range and high sensitivity has significant potential for future health monitoring and artificial intelligence applications.

Highly Sensitive Pressure Sensor Based on Elastic Conductive Microspheres

Elastic pressure sensors play a crucial role in the digital economy, such as in health care systems and human-machine interfacing. However, the low sensitivity of these sensors restricts their further development and wider application prospects. This issue can be resolved by introducing microstructures in flexible pressure-sensitive materials as a common method to improve their sensitivity. However, complex processes limit such strategies. Herein, a cost-effective and simple process was developed for manufacturing surface microstructures of flexible pressure-sensitive films. The strategy involved the combination of MXene-single-walled carbon nanotubes (SWCNT) with mass-produced Polydimethylsiloxane (PDMS) microspheres to form advanced microstructures. Next, the conductive silica gel films with pitted microstructures were obtained through a 3D-printed mold as flexible electrodes, and assembled into flexible resistive pressure sensors. The sensor exhibited a sensitivity reaching 2.6 kPa-1 with a short response time of 56 ms and a detection limit of 5.1 Pa. The sensor also displayed good cyclic stability and time stability, offering promising features for human health monitoring applications.

Biomimetic Wearable Sensors: Emerging Combination of Intelligence and Electronics

Owing to the advancement of interdisciplinary concepts, for example, wearable electronics, bioelectronics, and intelligent sensing, during the microelectronics industrial revolution, nowadays, extensively mature wearable sensing devices have become new favorites in the noninvasive human healthcare industry. The combination of wearable sensing devices with bionics is driving frontier developments in various fields, such as personalized medical monitoring and flexible electronics, due to the superior biocompatibilities and diverse sensing mechanisms. It is noticed that the integration of desired functions into wearable device materials can be realized by grafting biomimetic intelligence. Therefore, herein, the mechanism by which biomimetic materials satisfy and further enhance system functionality is reviewed. Next, wearable artificial sensory systems that integrate biomimetic sensing into portable sensing devices are introduced, which have received significant attention from the industry owing to their novel sensing approaches and portabilities. To address the limitations encountered by important signal and data units in biomimetic wearable sensing systems, two paths forward are identified and current challenges and opportunities are presented in this field. In summary, this review provides a further comprehensive understanding of the development of biomimetic wearable sensing devices from both breadth and depth perspectives, offering valuable guidance for future research and application expansion of these devices.

Biodegradable cotton fiber-based piezoresistive textiles for wearable biomonitoring

Electronic textiles are fundamentally changing the way we live. However, the inability to effectively recycle them is a considerable burden to the environment. In this study, we developed a cotton fiber-based piezoresistive textile (CF p-textile) for biomonitoring which is biocompatible, biodegradable, and environmentally friendly. These CF p-textiles were fabricated using a scalable dip-coating method to adhere MXene flakes to porous cotton cellulose fibers. The adhesion is made stronger by strong hydrogen bonding between MXene flakes and hierarchically porous cotton cellulose fibers. This cotton-fiber system provides a high sensitivity of 17.73 kPa^-1 in a wide pressure range (100 Pa-30 kPa), a 2 Pa subtle pressure detection limit, fast response/recovery time (80/40 ms), and good cycle stability (over 5, 000 cycles). With its compelling sensing performance, the CF p-textile can detect various human biomechanical activities, including pulsation, muscle movement, and swallowing, while still being comfortable to wear. Moreover, the cotton cellulose is decomposed into low-molecular weight cellulose or glucose as a result of the 1,4-glycosidic bond breakage when exposed to acid or during natural degradation, which allows the electronic textile to be biodegradable. This work offers an ecologically-benign, cost-effective and facile approach to fabricating high-performance wearable bioelectronics.

All-Nanofiber Network Structure for Ultrasensitive Piezoresistive Pressure Sensors

Sensing materials with fiber structures are excellent candidates for the fabrication of flexible pressure sensors due to their large specific surface area and abundant contact points. Here, an ultrathin, flexible piezoresistive pressure sensor that consists of a multilayer nanofiber network structure prepared via a simple electrospinning technique is reported. The ultrathin sensitive layer is composite nanofiber films composed of poly (3,4-ethylenedioxythiophene):poly (styrenesulfonate) and polyamide 6 (PEDOT:PSS/PA6) prepared by simultaneous electrospinning. PEDOT:PSS conductive fibers and PA6 elastic fibers are interwoven to form a multilayer network structure that can achieve ultrahigh sensitivity by forming a wealth of contact points during loading. In particular, gold-deposited PA6 fibers as upper and lower flexible electrodes can effectively increase the initial resistance. Due to this special fiber electrode structure, the sensor is able to generate a large electrical signal variability when subjected to a weak external force. The devices with different sensing properties can be obtained by controlling the electrospinning time. The sensor based on the PEDOT:PSS/PA6 nanofiber network has high sensitivity (6554.6 kPa^-1 at 0-1.4 kPa), fast response time (53 ms), and wide detection range (0-60 kPa). Significantly, the device maintains ultrahigh sensitivity when cyclically loaded over 10,000 cycles at 5 kPa, which makes it have great prospects for applications in human health monitoring and motion monitoring.

Graded Microstructured Flexible Pressure Sensors with High Sensitivity and an Ultrabroad Pressure Range for Epidermal Pulse Monitoring

Precisely detecting epidermal pulse waves with pressure sensors is crucial for pulse-based personalized health-monitoring technologies. However, developing a pressure sensor that simultaneously demonstrates high sensitivity and an ultrabroad pressure range and a convenient fabrication process for large-scale production is a considerable challenge. Herein, by utilizing a commercial conductive fabric (CF) and a silica gel film, we develop a high-performance pressure sensor (HPPS) for the monitoring of human physiological signals. Based on convenient turnover formwork technology, the silica gel film was fabricated by replicating the microstructure of the sandpaper surface. This microstructure and the plain weave structure on the CF surface together provide a sharp increase in the contact-separation area and structural compressibility, which are beneficial for the enhancement of output performance. Made of these two materials, the graded microstructured HPPS holds high sensitivity (4.5 mV/Pa), an ultrabroad pressure range (0-30 kPa), a wide working frequency bandwidth (up to 35 Hz), decent stability (>50,000 cycles), and a simple fabrication process that is suitable for large-scale production. Given these noticeable features, the developed HPPS not only succeeds in precisely detecting subtle pulse waves on various positions of different people but can also objectively capture changes in cardiovascular parameters caused by exercise training at different intensities in real time. These findings exhibit the enormous potential application of HPPS in tracking an individual's health status and comprehensively evaluating exercise intensity.

Recent Progress in Development of Wearable Pressure Sensors Derived from Biological Materials

This review summarizes recent progress in the use of biological materials (biomaterials) in wearable pressure sensors. Biomaterials are abundant, sustainable, biocompatible, and biodegradable. Especially, many have sophisticated hierarchical structure and biological characteristics, which are attractive candidates for facile and ecologically-benign fabrication of wearable pressure sensors that are biocompatible, biodegradable, and highly sensitivity. The biomaterials and structures that use them in wearable pressure sensors that exploit sensing mechanisms such as piezoelectric, triboelectric, piezoresistive and capacitive effects are present. Finally, remaining impediments are discussed to use of biomaterials in wearable pressure sensors.

Hierarchically Microstructure-Bioinspired Flexible Piezoresistive Bioelectronics

The naturally microstructure-bioinspired piezoresistive sensor for human-machine interaction and human health monitoring represents an attractive opportunity for wearable bioelectronics. However, due to the trade-off between sensitivity and linear detection range, obtaining piezoresistive sensors with both a wide pressure monitoring range and a high sensitivity is still a great challenge. Herein, we design a hierarchically microstructure-bioinspired flexible piezoresistive sensor consisting of a hierarchical polyaniline/polyvinylidene fluoride nanofiber (HPPNF) film sandwiched between two interlocking electrodes with microdome structure. Ascribed to the substantially enlarged 3D deformation rates, these bioelectronics exhibit an ultrahigh sensitivity of 53 kPa^-1, a pressure detection range from 58.4 to 960 Pa, a fast response time of 38 ms, and excellent cycle stability over 50 000 cycles. Furthermore, this conformally skin-adhered sensor successfully demonstrates the monitoring of human physiological signals and movement states, such as wrist pulse, throat activity, spinal posture, and gait recognition. Evidently, this hierarchically microstructure-bioinspired and amplified sensitivity piezoresistive sensor provides a promising strategy for the rapid development of next-generation wearable bioelectronics.